Put the potatoes and 2 tablespoons of salt in a large saucepan, cover with water and bring to a boil. Simmer the potatoes over moderate heat until tender, about 25-30 minutes. Drain the potatoes in a colander, then place the colander with the potatoes over the empty pot and cover with a clean, dry kitchen towel. Allow the potatoes to steam for 15 to 20 minutes.

Meanwhile, in a small bowl, whisk together the mayonnaise, yogurt, vinegar, Dijon mustard, whole grain mustard, herbs, and salt and pepper to taste (approx. 1/2 -1 tsp of each). Set aside.

When the potatoes are cool enough to handle, cut them into 2 inch chunks. Place the cut potatoes in a large bowl. While the potatoes are still warm, pour enough dressing over them to moisten well. Add the celery (if using) and red onion. Toss well, cover, and refrigerate for a few hours or overnight. Serve cold or at room temperature.